A facile route to ruthenium-carbene complexes and their application in furfural hydrogenation

Article abstract of DOI:10.1002/aoc.1584

A number of new N-heterocyclic carbene (NHC) ligands were synthesized via a multicomponent reaction, wherein an aldehyde or ketone, a primary amine and an a-acidic isocyanide were reacted, giving the corresponding 2H-2-imidazolines. These were easily alkylated with an alkyl halide at position N-3, yielding the final NHC precursors, that were then complexed with Ru in situ. The resulting complexes are shown to be active and selective catalysts for the transfer hydrogenation of furfural to furfurol, using isopropanol as the hydrogen source. Importantly, the carbene ligand remains coordinated to the ruthenium center throughout the reaction. Copyright

Full text of DOI:10.1002/aoc.1584

Letter to the Editor
Received: 9 July 2009
Revised: 5 October 2009
Accepted: 13 October 2009
Published online in Wiley Interscience: 8 December 2009
(www.interscience.com) DOI 10.1002/aoc.1584
A facile route to ruthenium–carbene
complexes and their application in furfural
hydrogenation
Zea Strassbergera, Maurice Mooijmanb, Eelco Ruijterb, Albert H. Albertsa,
Corien de Graaffb, Romano V. A. Orrub and Gadi Rothenberga∗
A number of new N-heterocyclic carbene (NHC) ligands were synthesized via a multicomponent reaction, wherein an aldehyde
or ketone, a primary amine and an α-acidic isocyanide were reacted, giving the corresponding 2H-2-imidazolines. These were
easily alkylated with an alkyl halide at position N-3, yielding the final NHC precursors, that were then complexed with Ru
in situ. The resulting complexes are shown to be active and selective catalysts for the transfer hydrogenation of furfural to
furfurol, using isopropanol as the hydrogen source. Importantly, the carbene ligand remains coordinated to the ruthenium
c
center throughout the reaction. Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: N-heterocyclic carbenes; multicomponent reaction; transfer hydrogenation; homogeneous catalysis
Introduction
donor, was then added, together with 1 equivalent of potassium
hydroxide, activating the catalyst. Then, 25 equivalents of furfural
wereadded,themixtureheatedto60 ◦Candstirredundernitrogen
for24 h.ReactionprogresswasmonitoredbyGC(seeExperimental
section for details).
One thorny problem in applied organometallic chemistry is
setting up a general protocol for ligand synthesis and complex
formation.[1,2] This is especially true in the case of carbene
complexes,[3,4] where synthesizing ligand backbones from scratch
can be a nightmare. This is a pity, since in many cases, and
particularlywithN-heterocycliccarbenes(NHCs), theresultingpre-
catalysts are air- and moisture-stable.[5–9] In this communication,
we present a simple, straightforward and inexpensive synthesis
route to NHCs and demonstrate the application of their ruthenium
derivatives in catalytic transfer hydrogenation. The synthesis
is based on a so-called multicomponent reaction (MCR).[10–18]
Importantly, this approach is general, giving access to a variety of
potential carbene ligand structures.
In a typical ligand synthesis (Fig. 1), an aldehyde or ketone
I, a primary amine II and an α-acidic isocyanide III were
combined in a one-pot reaction to give the corresponding 2H-2-
imidazoline. This was then easily alkylated with an alkyl halide IV at
position N-3, giving the final NHC precursor.[10,11] We synthesized
10 different catalyst precursors this way (structures 1–10 in
Scheme 1), all of which were isolated and characterized. These
precursors were then complexed with ruthenium and tested in
the transfer hydrogenation of furfural 11 to furfurol 12, using
isopropanol as the sacrificial hydrogen source [equation (1)].[19]
The deprotonation was done by mixing equivalent amounts of the
imidazoliniumprecursorandpotassiumtert-butoxidefor30 minat
40 ◦C. Subsequently, the resulting carbene was coordinated in situ
to [RuCl2(p-cymene)]2. The formation of the complex was shown
clearly by 13C NMR. Note that characterizing this complex by NMR
requires rigorous exclusion of water and oxygen under glove box
conditions (see Experimental section and Supporting Information
for details). Both reactions were complete within 30 min. An excess
of isopropanol, used as both solvent and sacrificial hydrogen
4 mol% Ru-Ligand
4 mol% KOtBu
4 mol% KOH
O
OH
(1)
H
i-PrOH 3 mL
THF 5 mL
60°C, 24h
O
O
OH
O
Furfural 11
Furfurol 12
Table 1 shows the substrate conversion, product selectivity and
product yield. In the absence of any carbene ligand, the metal
precursor [RuCl2(p-cymene)]2 already gives 94% conversion, with
84% yield to furfurol after 24 h. Nevertheless, adding a carbene
ligand can raise both numbers to >99%. Control experiments
showed that the hydrogen transfer reaction is sensitive to the type
and amount of base used.[7] The best conversions were obtained
with a mix of potassium tert-butoxide for deprotonating the
imidazolinium precursor, and KOH as a promoter.[20] No reaction
occurred in the absence of KOH, and <80% yield was observed
when using 0.1 equivalents of KOH with respect to the Ru catalyst.
∗
Correspondence to: Gadi Rothenberg, University of Amsterdam, Van ‘t Hoff
Institute for Molecular Sciences, Niewue Achtergracht 166, Amsterdam 1018
WV, The Netherlands. E-mail: g.rothenberg@uva.nl
a
Van ‘t Hoff Institute of Molecular Sciences, University of Amsterdam, Nieuwe
Achtergracht 166, 1018 WV Amsterdam, The Netherlands
b
Department of Organic and Inorganic Chemistry, Faculty of Science, Vrije
Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The
Netherlands
c
Appl. Organometal. Chem. 2010, 24, 142–146
Copyright ꢀ 2009 John Wiley & Sons, Ltd.

A facile route to ruthenium–carbene complexes and their application in furfural hydrogenation
(I)
O
(II)
R1 NH2
R2
R3
Cl
O
O
O
NH2
NH2
H
NH2
H
H
H
O
4 mol% [RuCl2(p-cymene)]2
4 mol% KOtBu
4 mol% KOH
R1
R2
R3
R1
N
R2
R3
N
N
R
Ru
4
N
R6
R4
R5
I
R
i-PrOH 3 mL
THF 5 mL
60°C, 24h
5
NHC-Ru catalysts
R6
(IV)
(III)
NC
R5
R6
X
R4
O
Br
MeI
Cl
O
Cl
O
O
MeO
O2N
NC
NC
MeO
Cl
Br
Br
Cl
O
Cl
Br
OMe
Figure 1. MulticomponentsynthesisroutetoN-heterocycliccarbeneligandprecursors.Notethatonlyasmallselectionofthe1080precursorpermutations
were synthesized.
OMe
Cl
N
N
N
I
I
I
N
N
N
N
O
I
N
O
MeO
NO2
O
1
2
3
4
OMe
Cl
Cl
OMe
OMe
N
N
N
N
N
O
O
I
I
I
O
N
OMe
MeO
MeO
O
5
6
7
N
N
N
N
I
I
I
N
N
OMe
O
8
9
10
Scheme 1. The 10 new N-heterocyclic carbene precursors synthesized via the multicomponent route.
c
Appl. Organometal. Chem. 2010, 24, 142–146
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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Z. Strassberger et al.
ticanalysisisbeyondthescopeofthispreliminarycommunication,
examining the structures in Scheme 1 reveals an interesting re-
lationship: structures 2, 3 and 9 all have the rigid spiro-bicyclic
moiety, plus large hindering groups at the N-1 and N-3 positions. A
simple molecular mechanics simulation shows that the ruthenium
centre in all these three cases is effectively ‘hidden’, resulting in
a narrow reaction pocket.[21] Conversely, structures 5, 6 and 8 all
lack the spiro construction and have a small and flexible oxygen-
bearing group on the N-3 position. Competitive coordination by
this oxygen to the Ru atom may explain the lower conversion
obtained with these complexes.[22] Once the furfural coordinates,
however, the reaction proceeds with a comparable selectivity to
the other complexes.
One question that springs to mind in such systems pertains
to the ligand dissociation equilibria. Namely, what is the chance
that, in the course of the catalytic cycle, the carbene ligands
dissociate from the Ru complex? Happily, we can say that
our carbene ligands remain coordinated throughout the cycle.
Any free carbene in solution would lead to immediate acyloin
condensation, analogous to the reactions reported by Enders
et al.[23] A series of separate control experiments showed that
this acyloin condensation [equation (2), giving roughly 90%
furoin 13 and 10% furyl 14] is very fast under our reaction
conditions compared with the transfer hydrogenation reaction.
Thus, using a 1 : 1 ligand : metal ratio, we managed to avoid any
acyloin condensation, confirming that the carbene ligand remains
coordinated to the ruthenium complex throughout the catalytic
cycle.
Table 1. Ru–carbene catalyzed transfer hydrogenation of furfural to
furfurola
Furfurol
Imidazolinum
ligand
precursor
Furfural
Selectivity
(%)b
Yield
(%)c
Entry
conversion (%)a
1
1
92.94
95.34
99.61
99.91
81.04
63.50
98.72
63.32
98.05
99.48
94.15
99.00
98.32
99.40
98.98
99.54
98.99
98.37
99.22
99.01
98.88
90.10
92.01
93.74
99.01
98.89
80.67
62.86
97.11
62.83
97.08
98.37
84.63
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
11
No ligand
a Standard reaction conditions: 5 mmol furfural 11, 0.1 mmol [RuCl2(p-
cymene)]2; 0.2 mmol 2-imidazolinium precursor (structures shown in
Scheme 1);0.2 mmolKOtBu;0.1 mmolKOH;5 mldriedTHF;3 mli-PrOH;
N2 atmosphere; 60 ◦C; 24 h.
b Determined dividing the total amount of furfurol 12 by the total
amount of products.
c Determined by GC analysis using n-octane as internal standard.
100
90
80
70
60
50
40
3 mol% 1-Methyl-3-
octylimidazolium
tetrafluoroborate
3 mol% KOtBu
H
O
THF 50 mL, 24 °C
O
(2)
11
O
O
O
O
O
L3
L6
L9
L1
O
L2
L5
L8
30
20
10
0
OH
O
L4
Furoin 13
Furyl 14
L7
L10
Theoretically, a reductive elimination of an intermediate (di)-
hydride to an imidazole species that does not catalyze acyloin
condensation is also possible, but since our reactions are under
basic conditions, the base would restore the carbene in situ. Thus,
any carbene coming off as an H-imidazole would be restored
and then catalyze the above acyloin condensation. Since this
condensation was not observed, and since a series of control
experiments confirmed that neither OH− nor t-BuO− in itself
promotes the acyloin condensation of furfural, we maintain that
indeed the Ru remains coordinated to the carbene.
In conclusion, we show that the multicomponent reaction
of aldehydes/ketones, primary amines and α-acidic isocyanides
opens a facile and effective route to imidazolinium salts.
Ruthenium–carbene complexes based on such salts are good
transfer hydrogenation catalysts. The fact that the carbene ligand
remains coordinated to the ruthenium centre throughout the
reaction creates an opportunity for ligand design, and indeed
ligands with different peripheral groups show markedly different
catalysis effects.[24]
0
5
10
Time /h
15
20
Figure 2. Time-resolved profiles for the Ru-carbene catalyzed hydrogena-
tion of furfural 11 to furfurol 12. Reaction conditions are as in Table 1.
The broken curve shows the blank experiment using [RuCl2(p-cymene)]2
precursor only (no carbene ligand).
Figure 2 shows the time-resolved profiles for the transfer
hydrogenation reaction catalyzed by the 10 different ruthe-
nium–carbene complexes made from the imidazolinium pre-
cursors in Scheme 1, where Ln denotes the ligand prepared using
precursor n. For comparison, the broken curve shows the yield
obtained without a carbene ligand {i.e. using only the [RuCl2(p-
cymene)]2 precursor}. We see that several of the complexes, and
especially 2, 3 and 9, exhibit both fast and highly selective catal-
ysis. Curiously, the catalysts based on precursors 5, 6 and 8 are
poorer than the carbene-free complex. While a detailed mechanis-
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 142–146

A facile route to ruthenium–carbene complexes and their application in furfural hydrogenation
Experimental Section
126.49 (CH), 114.22 (CH), 114.11 (CH), 85.30 (C), 84.32 (C), 72.08
(CH3), 68.63 (CH3), 55.28 (CH3), 55.26 (CH3), 53.05 (CH3), 48.89
(CH2), 48.35 (CH2); IR: 3064.03, 3020.63, 2942.02, 2912.13, 2838.35,
1722.97, 1608.69, 1511.76, 1490.06, 1430.75, 1360.34, 1290.90,
1232.55, 1170.83, 1087.89, 1014.11, 933.58, 695.84638.46; HRMS
calculated for C25H24ClN2O3 (M+ + H) 435.1470, found 434.1454.
The synthesis details and characterization data for archetypes B,
C, and D are provided in the Supporting Information.
Materials and Instrumentation
GC analysis was performed on an Interscience GC-8000 gas
chromatograph with a 100% dimethylpolysiloxane capillary
column (VB-1, 30 m × 0.325 mm). Samples for GC analysis were
diluted in 1◦ml EtOH. GC conditions: isotherm at 60 ◦C (2 min);
ramp at 50 C min−1 to 80 ◦C; isotherm at 80 ◦C (3 min); ramp
at 1 ◦C min−1 to 90 ◦C; ramp at 50 ◦C min−1 to 250 ◦C; isotherm
at 250 ◦C (3 min). 1H NMR spectra were recorded on a Varian
Mercury 300 (300 MHz, CDCl3) All reactions were performed
under N2 using standard Schlenk techniques. Unless otherwise
noted, all chemicals were purchased from commercial sources
and used as received. All products are known compounds and
were identified by comparing of their GC retention times and/or
NMR spectra to those of authentic samples. Ligand 10 is a
known compound, and was synthesized following a published
procedure.[17] Ligands 1–9 are new compounds, synthesized
using a modified procedure as outlined below. Detailed synthesis
procedures and characterization data for these compounds, as
well as details of the NMR characterization control experiments,
are included in the Supporting Information.
Procedure for the Synthesis of 2-Imidazolinium Salts
Reactions were carried out at a concentration of 1 M of imidazoline
in acetone. The halide (1 equiv.) was added to a stirred solution of
the 2H-2-imidazoline and NaI (1 equiv.). The reaction mixture was
stirred at rt for 18 h. Then, the reaction mixture was filtrated over
celite and concentrated in vacuo.
Example 1: imidazolium iodide 1
N
I
General Procedure for Synthesizing the 2H-2-imidazoline
Archetypes
N
O
Fourarchetypesof2H-2-imidazolinesweresynthesizedviaathree-
component Mannich-type reaction. All reactions were performed
using a concentration of 1 M of aldehyde, 1 M of amine, and 0.5 M
of isocyanide in dry CH2Cl2 or MeOH. Na2SO4 and the aldehyde
were added, at 25 ◦C, to a stirred solution of the amine. The
mixture was stirred for 2 h. The isocyanide was then added and
the mixture was stirred at 25 ◦C for an additional 18 h, filtered and
concentrated in vacuo. The crude product was purified by flash
column chromatography (cyclohexane–EtOAc–Et3N = 2 : 1 : 0.01,
gradient, unless stated otherwise).
O
According to general procedure II for the synthe-
sis of 2-imidazolinium salts, the reaction between 2H-2-
imidazoline D (276.2 mg, 1.00 mmol), NaI (149.9 mg, 1.00 mmol)
and 1-(chloromethoxy)-2-methoxyethane (123.0 mg, 112.8 µl,
1.00 mmol) afforded 2-imidazolinium iodide
1 (482.1 mg,
0.979 mmol, 98%) as a yellow foam. 1H NMR (500 MHz, CDCl3)
δ 9.794 (s, 1H), 7.74 (d, J = 7.2, 2H), 7.69 (d, J = 7.2, 2H), 7.52–7.48
(m, 2H), 7.44–7.40 (m, 2H), 4.832 (s, 2H), 4.291 9s, 2H), 3.44 (t,
J = 1.5, 2 H), 3.207 (s, 3 H), 3.16 (t, J = 1.5, 2 H), 1.02 (s, 9 H); 13C
NMR (125 MHz, CDCl3) δ: 156.98 (CH), 142.35 (C), 140.12 (C), 130.99
(CH), 129.24 (CH), 124.59 (CH), 120.77 (CH), 76.28 (CH2), 73.48
(C), 71.12 (CH2), 68.70 (CH2), 58.90 (CH3), 58.57 (C), 57.81 (CH2),
28.02 (CH3); IR: 2982.53 (m), 2916.95 (m), 2876.92 (m), 1624.12 (s),
1448.11 (m), 1295.24 (br), 1229.18 (m), 1181.92 (m), 1095.60 (m),
1032.06 (br), 844.85 (m), 767.69 (s), 756.12 (s), 734.42 (s); HRMS
calculated for C23H29N2O2 (M+ − I−) 365.2197, found 365.2210.
Example: 2H-2-imidazoline type A
OMe
Cl
N
O
N
MeO
Procedure for Preparing the Ru–Carbene Complexes
Starting from p-methoxybenzylamine (177.9 mg, 1.298 ml,
10.0 mmol), p-chlorobenzaldehyde (1.405 g, 10.0 mmol) and
methyl 2-isocyano-2-phenylacetate (876 mg, 749 µl, 5.00 mmol)
yielded 2H-2-imidazoline A (339.2 mg, 7.80 mmol, 78%) as 3 : 2
mixture of diastereoisomers as a yellow foam. 1H NMR (250 MHz
CDCl3) δ: 7.62–7.60 (m, 2H), 7.37–7.29 (m, 4H), 7.25–7.21 (m, 2H),
7.06–6.97 (m, 7H), 6.94–6.83 (m, 7H), 6.76 (m, 2H), 5.272 (s 1H),
4.729 (s, 1H), 4.35 (d, J = 14.7, 2H), 4.31 (d, J = 14.8, 2H), 3.81–3.25
(m, 4H), 3.811 (s, 3H), 3.767 (s, 3H), 3.728 (s, 3H), 3.256 (s, 3H); 13C
NMR (125 MHz CDCl3) δ: 173.82 (C), 170.97 (C), 159.37 (C), 159.26
(C), 156.63 (CH), 155.64 (CH), 142.95 (C), 137.07 (C), 135.22 (C),
134.30 (C), 133.95 (C), 133.24 (C), 129.52 (2 × CH), 129.45 (CH),
129.11 (CH), 128.78 (CH), 128.26 (CH), 127.98 (2 × CH), 127.72 (CH),
127.71 (CH), 127.31 (C), 127.25 (CH), 127.06 (C), 126.53 (2 × CH),
A solution of [RuCl2(p-cymene)]2 (61 mg, 0.1 mmol) was stirred
under N2 in a 25 ml Schlenk tube together with the 2-
imidazolinium salt (0.2 mmol), KOtBu (22 mg, 0.2 mmol) and KOH
(6 mg, 0.1 mmol) in 5.0 ml dried THF and 3.0 ml i-PrOH for 30 min
at 40 ◦C.
Example: [RuCl2(p-cymene)]2/1
[RuCl2(p-cymene)]2 (61 mg, 0.1 mmol), 2-imidazolinium salt 1
(73.1 mg, 0.2 mmol), KOtBu (22 mg, 0.2 mmol) and KOH (6 mg,
0.1 mmol) were stirred in 5.0 ml dried THF and 3.0 ml i-PrOH for
30 min at 40 ◦C.
c
Appl. Organometal. Chem. 2010, 24, 142–146
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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Z. Strassberger et al.
Procedure for Catalytic Transfer Hydrogenation of Furfural
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2009, 131, 4584.
In a 25 ml Schlenk tube under N2, the catalyst was first prepared
according to the above procedure. Furfural 11 (480 mg, 5.0 mmol)
and octane (240 mg) were added and the reaction mixture was
heated to 60 ◦C and stirred for 24 h. Periodically, samples were
taken out and analyzed by GC.
[3] M. R. Eberhard, B. van Vliet, L. Dura´n Pa´chon, G. Rothenberg,
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2009, 1313.
[4] J. J. L. M. Cornelissen, R. van Heerbeek, P. C. J. Kamer, J. N. H. Reek,
N. A. J. M. Sommerdijk, R. J. M. Nolte, Adv. Mater. 2002, 14, 489.
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[7] S. Warsink, P. Hauwert, M. A. Siegler, A. L. Spek, C. J. Elsevier, Appl.
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Procedure for Acyloin Condensation of Furfural 11
to Furoin 13
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[9] D. Gnanamgari, E. L. O. Sauer, N. D. Schley, C. Butler, C. D. Incarvito,
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[11] R. S. Bon, N. E. Sprenkels, M. M. Koningstein, R. F. Schmitz, F. J. J. de
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[13] A. Do¨mling, Chem. Rev. 2006, 106, 17.
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R. V. A. Orru, J. Org. Chem. 2007, 72, 6135.
In a 100 ml Schlenk tube equipment under N2, 1-methyl-3-
octylimidazolium tetrafluoroborate (28 mg, 1 mmol) and KOtBu
(122 mg, 1 mmol) were dissolved in 50 ml of dry THF and
stirred for 15 min at 24 ◦C. After addition of distilled furfural
11 (3200 mg, 33 mmol) the reaction mixture was stirred overnight,
and quenched with 2 ml formic acid 98%. The solvents were
evaporated and the crude product was recrystallized from EtOH
(85 ml), to give 13 as a yellow solid (3030 mg, 31.56 mmol, 94%)
containing 2-3% of furyl 14 as side product.
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C. V. Stevens, M. Swart, F. M. Bickelhaupt, M. B. Groen, R. V. A. Orru,
J. Org. Chem. 2005, 70, 3542.
Procedure for Characterization of the Ru–Carbene Complex
by 13C NMR
In a glovebox, a flame-dried NMR tube containing a solution of
imidazolium salt 1 (30.0 mg, 60.8 µmol, 1 equiv.) and [RuCl2(p-
cymene)]2 (37.7 mg, 60.8 µmol, 1 equiv.) in THF (1 ml) was
prepared. Subsequently, KOtBu (7.0 mg, 60.8 µmol, 1 equiv.) was
added. The resulting reaction mixture was stirred on a vortex mixer
for 1 min and subsequently 1H and 13C NMR measurements were
performed. In the 13C NMR analysis the C2-Ru signal was observed
at 166.07 ppm as a small peak.
[18] N. Elders, E. Ruijter, F. J. J. de Kanter, M. B. Groen, R. V.A. Orru, Chem.
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2008, 10, 608.
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Supporting information
Supporting information may be found in the online version of this
article.
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c
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Appl. Organometal. Chem. 2010, 24, 142–146